Composite bituminous pavement and method of laying the same



June 18, I M. GORDON COMPOSITE BITUMINOUS` PAVEMENT AND METHOD OF LAYING THE SAME Filed June 21, 1926 INVENToR.

BY Zn l r i: i :wwwf ATTORNEYS.

generally improve and Patented June 18, 1929. p

UNITED STATES ,PATENT OFFICE.

MORRIS l L.' GORDON, OF BUBLINQAHE, CALIFORNIA. COMPOSITE BITUIIIOUS PAVEMENT AND HETHOD 0I LAYING THE appmmtmninea :une 21,

This invention relates to a composite bi ous pavement and the method of laying the same. s

Practically all bituminous pavements constructed and in use today consist of two courses of material--a lower or Ybase course and an upper course or wearing surface.

The lower course usually consists of coarse' crushed rock and a filler of tglr gate and sand combined wi a suiiiclent amount of bituminous binder to coat all themater'ial. The upper course usually consists of line crushed aggregate and sand, together with a bituminous binder, and in rsome instances of sand and a bituminous binder only. The two courses are spread hot and rolled and to all appearances when completed produce a pavement of considerable stability and with wearresisting quahties. N umerous, defects however, usually develop in a pavement of this character in a comparatively short period of time. When the pavement is subjected to heavy traiiic during hot weather, the wearing surface becomes soft and creeps, thereby producing a wa or corrugated surface; the surface layer becomes thm in spots and soon wears through. The coarser rocks in the lower layer begin to shatter or abrade; water vbegins to penetrate and general disintegration takes place.

The object of the present invention is to simplify the construction of pavements of the character described; tovprovide a wear-resisting surface which will not creep; to provide a pavement which is resilient and shock-absorbing in its upper portion; to provide a pavement with greater stability and compactness to resist displacement by trac, and further t0 provide a method of laying the pavement whereby greater compactness is obtained and better unity and bond is secured between the several courses forming the pavement.

For convenience in describing the construction of the pavement forming the subject matter of the present invention, and the method of preparing and laying the same, reference will be made to the accompanying drawing `which shows a vertical cross-section of a portion of the pavement. By reference to the drawing it will be noted that the pavement is laid in three courses-a bottom aded aggreor lower course, a, an intermediate course,

-of a line mineral b, and a top course o r wearing surface, c. The upper course 1s composed chlefly matter, rich in ybituminous `coarse broken rock, or

ma semi 1ro. 117,409.

binder to effectively resist abrasive action and suction from fast-moving, rubber-tired traic. The intermediate layer forms a sup-l port for the above surface and is composed chleiiy of a slightly coarser mineral matter containing a somewhat less vquantity of bituminous binder.v The intermediate layer. also forms a covering for the lower layer and protects the coarser stones in the lower layer, and furthermore assists in producing stability throughout the entire pavement. The lower course is composed chiefly of the like, and a filler consisting of a mixture of bituminous binder mixed with a mineral matter. This course may be laid in any thickness from one inch to four inches. No coarse rock should, however, be employed which has a diameter that will equal more than three-fourths the of the course. For instance, if the course is four inches thick, the coarse rock may be as large as three inches. The mineral aggregate which forms the liller between Vthe thereof consists of crushed rock or material which passes a one-half inch screen, another portion passing a one-fourth inch screen, another portion a tenmesh screen, and still another ortion will pass a 200- mesh screen. To t is mineral a gregate is added from 2 to 7% -by weight of ituminous binder. The aggregate and the binder, togetherwith the coarse rock, is run through dryer and a mixer where it is thoroughly heated and mixed with the binder, the aggregate, and the rock. The above mixture should be delivered to the roadway to be pavedin a thoroughly heated condition and there dumped in the most convenient manner for handling. After dumping on the sub: rade indicated at D, it is spread out with got shovels or forks and when the required thickness is obtained it should be immediately rolled with a heavy roller, the rollin to be continued until the course has receive about 75% of its final compression and while still hot it should be immediately covered with a second course mixture. s

The second -or intermediate course con- 'sists chieiy of a'crushed rock or like mathickness f tity of the mixture may be added to ring the surface to a true and even contour.-

After all low places have been filled 'in and thoroughly compressed and while still hot, the pavementis ready for the final surface course. This vmay consist of' bituminous binder and sand alone, or it may Aconsist of crushed material passing a 1/4 screen and mixed with' finer material passing a 200- mesh screen, this graded ag egate being then mixed with 10 lto 18% y weight of bituminous binder. This mixture `is. also heated and thoroughly mixed and is delivered to the pavement at a temperature ranging from 250 to 500 F. Itis here spread to the required thickness and immediately rolled, the rolling being continued until no further compression may be obtained. In actual practice, where the total thickness of the pavement when completed is five inches, the up er course will not exceed one-fourth of an inch, the intermediate course willbe approximately three-fourths of an inch, and the lower course four inches. All of the courses in this pavement can be roduced by an ordinary paving plant designed to prepare hot mixed coarse aggregate pavements without additions or alterations. The mineral filler in all mixtures may be added cold, while tins-"other ingredients are being mixed if such method 1s more vconvenient providingtethe ingredients are suiiicientl1 eated a r the cold ller has been adde ment should be. soluble in cold carbon bisulphide, and it may be rened from tar,

l e determined for each individual natural asphalt or petroleum oils. The

netration or hardness of the binder should g paving job after considering climatic conditions at the local point whereI the mixture is to be used for pavin The methodof laying the pavement is a so an important feature of the present application. In actual practise, it has been found that when the rst'course or layer of material vis placed and rolled that chilling ofthe lower surface takes place due to contact with the sub-grade, and similarly that the upper contact wit the entire body of material is not compressed to as high` a `compactness as might bedesired, the center portiononly of the course reachin a proper com actness as it retains its semilastic condition for the longest periody o time. When the second or intermediate course is laid, -it has a temperature surface chills* rapidly due 'to theroller. As a consequence,v

of from 250 to 500 'The temperature is uickly conducted to the upper surfaceY of t e lower course and the surface is thus softened so that compactness is .increased when the intermediate course is rolled. The heating of the upper surface of the lower course to increase its compactness when rerolling is important, and re-heating is also important as it produces a closer unity or bond between the two layers., p f,

The upper course when finally laid also has a high temperature of from 250 to 500 F., and as such reheats the upper portion of the intermediate layer. This is also important as the compactness thereof is increased when the upper course is rolled and a better bond is obtained between the intermediate and upper course. In fact the en tire compactness and stability of the Pavement is materially increased by laying the several courses as described.

A pavement laid byv this method is accordingly not only compact throughout but also contains great stability under traic due to the bond formed by the layers and the interlocking eifect obtained by the gradualV grading of the aggro ate with relation to the coarse rock in the ower layer.`

The gradual grading of the mineral matter from coarse rock to fine is of further importance. For instance when the first layer or lower course is lai and rolled a comparatively rough or uneven surface is obtained. If a wearing surface was placed directly thereon, it would be uneven in thickness and when subjected to heavy trailic in warm weather itwould soon creep and be ycome corduroyd. Not so in this instance.

The rough surface obtained after rollin of the first course is covered by the interme 'ate layer and as this contains a comparatively iine aggregate or crushed rock, it may be raked and rolled to a substantially smooth surface or contour and as such presents an ideal surface for reception of the upper layer or wearing surface. This up er layer will accordingly be uniform in thic ness and ma be made exceedingly thin. Asalready state a quarter of an inch thickness -is more than f sufficient and maybe laid even thinner if desired. This is ,'a decided advantage, as e rience has shown thata thin surface of umform thickness has comparatively little,

if any, tendency to creep. B usm the proportions of inlients and y fol owin the method descri` such as locating the iiferent sizes of mineral aggregate 1n their proper places, a pavement is produced which is substantially monolithic in structure. The body of the pavement has great compactness and stability and two u perllayers are formed which -are not o y wear-resisting and shock-absorbing, but which will also render the pavement impervivous to moisture and disintegration.

kgate which should comply:-

When laying a avement which is five inches thick, Il pre er to use crushed rock and aggregates in substantially the following proportions z In the lower course the crushed rock aggregates and fine material will conform to Substantially the following grading: first,- crushed rock not over 3 in size; retained upon a l screen, to 65%; passing a 1/2 screen and retained upon a 1/1 screen 8 to 20%; passing a 1%, screen and retained upon a lO-mesh screen, 8 to 15%' passing the 10-mesh screen, 8 tc 28%. Of the material passing the 10-mesh screen, at least 8% should pass a 200mesh screen. To this is added 2 to 7% by weight of bituminous binder.

The-second course should comply to the following grading: crushed rock or like material retainedupon a y2 screen not to` exceed 5%; passing the 1/2 screen and retained upon a 1%1 screen, 40 to 65%; passin the 2% screen and retained upon a 10-mes screen, 15 to 30%; passing the 10mesh screen and retained upon a 200mesh screen,

8 to 25%, and passin the 200mesh screen, 5 to 10%. To t is is a ded 4 to 8% by weight of bituminous inder.

The surface course is composed of aggrewith the following grading: crushed rock or like material retained upon a 1/1 mesh screen not to exceed 5%; passing the 10-mesh screen and retained upon a 200mesh screen, 55 to 75%, and passlng the 200mesh screen, 15 to 30%. To' this is added 10 to 18% by weight of bituminous binder.

When local material does not contain suliicient filler to conform to the above grading, Portland cement, lime dust, carbonated lime, or other manufactured filler of approved quality, may be used to make up the shortage. In'this specification, all material retained upon a 10-mesh screen is referred to as crushed rock or stone. All material passing the lO-mesh screen and retained upon a 200mesh screen is referred to as aggregate, and all material passing a 200mesh screen Asingle layer composed of is reerred to as line aggregate or filling materia Having thus described my invention, what I claim and desire to secure by Letters Patent isl 1. A method of constructing a bituminous pavement of the character described which comprises spreading a hot coarse aggregate bltuminous concrete mixture in a layer approximately the thickness desired for the complete pavement, compressing 4it to approximately seventy-five percent of the compression it will take and while still hot covering it with a hot iiner aggregate bituminous concrete mixture in a layer approximately one fourth the desired depth of the complete pavement, compressing it to approximately seventy-five percent of the compression it will take and while this. mixture is still hot covering it with a still ner aggregate bituminous mixture and compressing the complete pavement into a single layer.

2. A bituminous pavement consisting of a three dissimilar bituminous mixtures so disposed that the'lower three-fourths to four-fifths of the layer con.

sists of a dense bituminous concrete composed principall of mineral particles three lnches to one-hall? inch size; ninety-live percent of the upper one-fifth to one-fourth of the layer consisting of a finer aggregate bituminous concrete composed principally of mineral particles one-half inch and onefourth inch in size; and the uppermost live per cent of the layer consistin of a bituminous mixture in which all o? the mineral particles are less than one-half inch and at least ninety-five percent of them range from one-fourth inch size to filler which will pass a 200 mesh screen; the three mixtures being so combined during the process of laying and compressing that the result is a single layer in the upper one-fourth of which the predominating size of the particles of mineral decreases to a minimum and the per' centage of bituminous binder increases to a maximum at the p. e

ORRIS L. GORDON. 

